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Significance. Unobtrusive vital signs monitoring is of major importance for various medical areas such as detection and treatment of sleep disorders, monitoring neonates and burned victims, home health care and smart home applications and wearables among others. Such applications call for monitoring methods in which the patient’s natural state is less interfered with. An ideal device would be non-invasive, minimally restrictive, robust enough to compensate movements of the patients, and would operate without relying on the patient’s full cooperation. Objective. This paper focuses on the design and development of an unobtrusive vital signs monitoring system particularly suited for long-term monitoring placed under the mattresses. Approach. The system is based on the magnetic induction sensing method, designed to infer presence on the bed, breathing and cardiac activity, and consists of two coils for excitation and detection. The new detection coil is based on a concentric planar gradiometer for canceling the primary field. The signal acquisition system has been designed using simple electronics to avoid ending up with a complex and expensive system. The experimental results were compared with reference signals coming from other known sensors with different technical bases for benchmarking and identifying the advantages and/or drawbacks of the new system regarding other techniques. The designed system was also studied in regards to safety standards and limitations for the exposure to the magnetic fields. Main results. Experimental results confirm the suitability and safety of the sensor for long-term cardiac and respiratory monitoring. The system is able to detect respiration and cardiac activity as well as the presence on the bed and changes in position. [ABSTRACT FROM AUTHOR]